How to slice an object depending on whether it is in camera or not
With the help of rob I've figured out how to do it and to help people that have a similar question in the future I post this answer. It is made in a way so that it is fully scriptable, you can of course do it by hand.


Remodel blenders camera 
Set a real blender camera as parent object. This step can be done in a 
seperate file if you then append it.
Position the parent-camera however you like
Extrude the remodeled camera
Add a boolean modifier to the remodeled camera and select the other object 
that you want to cut out
Apply the modifier and delete all the stuff you don't need.


Important: For my project I only need the 3D shape, if you need the materials textures etc. you'd have to figure out how to also include it, because it won't be there after cutting (probably just add the boolean modifier to the car)